Electrical connector assemblies for making large numbers of interconnections are used extensively in the electrical connector industry, such as for use in computers and other electronic devices. With the ever-increasing miniaturization of the electronics in such devices and the ever-increasing density of the related connector assemblies, continuing problems occur in designing connectors for such use. This is particularly true with connectors commonly known as card edge connectors which are constructed to receive printed circuit cards having conductive pads on one or both sides of the card along the edge of the card which is inserted into the connector assembly.
One of the problems with card edge connector assemblies of the character described above is achieving a desired range of contact engaging forces between the contact elements of the connector assembly and the conductive pads on the printed circuit card. In most such connector assemblies, contact elements are arranged along opposite sides of the card receiving slot of a housing for the connector assembly. The contact elements engage conductive pads on opposite sides of the printed circuit board. Heretofore, a common thickness for a printed circuit card for use with card edge connector assemblies has been on the order of 0.062 inch. With the ever-increasing miniaturization of electronic systems utilizing these connector assemblies, it is desireable to reduce the thickness of such printed circuit cards. This, however creates problems in maintaining the desired range of forces between the contact elements and the conductive pads on the printed circuit card.
More particularly, it is known that the force generated by a spring is equal to the spring constant multiplied by the deflection of the spring (i.e., f=k.multidot.d). Using this formula, a desired normal force between the contact elements and the card conductive pads can be achieved with a spring contact having a long travel (i.e., deflection) or with a spring contact having a short travel (i.e., deflection). In the connector art, it is generally desirable to use contact elementswith a long travel (i.e., deflection) to achieve the desired range of forces because such a long travel will minimize the affect of any variations due to manufacturing tolerances. That is, for a given variation in travel due to tolerances, the percentage of variation will be smaller with a contact element that is deflected a greater distance. This commonly is termed a low spring rate system. Lower spring rates, in the context of electrical contact elements, normally are accomplished by providing long spring contacts. Of course, there are limitations placed on the contact structure by the miniaturization of the overall) connector assembly.
Heretofore, most attempts to provide a lower spring rate system in card edge connector assemblies of the character describedin order to achieve a desired range of contact forces have utilized preloaded contact elements. However, preloading the contact elements causes problems and/or creates limitations in fabricating and usage of the connector assemblies.
When a large number of contact elements are placed under a preload, stresses are placed on the housing itself which conventionally is fabricated of molded plastic material or the like. These preloading stresses place limitations on the types of plastic material that can be used in certain applications. For instance, it often is desirable to surface mount such connector assemblies on a printed circuit board. During such surface mounting procedures, the housing and contact elements are exposed to elevated temperatures of 220 degrees Centigrade. When certain plastics reach that temperature, and the plastic material is subjected to the forces of the preloaded contact elements, the plastic material has a tendency to creep. Consequently, preloading of contact elements in card edge connector assemblies has definite limitations.
This invention is directed to solving these problems by providing a card edge connector assembly in which the contact elements have a relatively low spring rate but are not preloaded.